Scanned display with pinch, timing, and distortion correction

ABSTRACT

A display apparatus includes an image source that scans about two axes. To offset motion about a first of the axes during sweeps about the second axis, the apparatus includes a structure to produce offsetting motion about the first axis at a scanning rate equal to the twice-scanning rate about the second axis. The offsetting scan can be a ramp or other motion. In one embodiment, the offsetting motion is a resonant sinusoid. The offsetting motion may be produced by an auxiliary scanner such as a mechanical scanner, a piezoelectric scanner, a MEMs scanner or other scanner. Because the offsetting motion is very small, the auxiliary scanner can function with a very small scan angle.

TECHNICAL FIELD

The present invention relates to scanned light devices and, moreparticularly, to scanned light beam displays and imaging devices thatproduce images for viewing or collecting images.

BACKGROUND OF THE INVENTION

A variety of techniques are available for providing visual displays ofgraphical or video images to a user. For example, cathode ray tube typedisplays (CRTs), such as televisions and computer monitors are verycommon. Such devices suffer from several limitations. For example, CRTsare bulky and consume substantial amounts of power, making themundesirable for portable or head-mounted applications.

Flat panel displays, such as liquid crystal displays and field emissiondisplays, may be less bulky and consume less power. However, typicalflat panel displays utilize screens that are several inches across. Suchscreens have limited use in head mounted applications or in applicationswhere the display is intended to occupy only a small portion of a user'sfield of view.

One approach to overcoming many limitations of conventional displays isa scanned beam display, such as that described in U.S. Pat. No.5,467,104 of Furness et al., entitled VIRTUAL RETINAL DISPLAY, which isincorporated herein by reference. As shown in FIG. 1, in a scanned beamdisplay 40, a scanning source 42 outputs a scanned beam of light that iscoupled to a viewer's eye 44 by a beam combiner 46. In scanned displays,a scanner, such as a scanning mirror or acousto-optic scanner, scans amodulated light beam onto a viewer's retina. An example of such ascanner is described in U.S. Pat. No. 5,557,444 to Melville et al.,entitled MINIATURE OPTICAL SCANNER FOR A TWO-AXIS SCANNING SYSTEM, whichis incorporated herein by reference. The scanned light enters the eye 44through the viewer's pupil 48 and is imaged onto the retina 59 by thecornea. In response to the scanned light the viewer perceives an image.

Sometimes such displays are used for partial or augmented viewapplications. In such applications, a portion of the display ispositioned in the user's field of view and presents an image thatoccupies a region 43 of the user's field of view 45, as shown in FIG.2A. The user can thus see both a displayed virtual image 47 andbackground information 49. If the background light is occluded, theviewer perceives only the virtual image 47, as shown in FIG. 2B.

One difficulty with such displays is raster pinch, as will now beexplained with reference to FIGS. 3-5. As shown diagrammatically in FIG.3, the scanning source 42 includes an optical source 50 that emits abeam 52 of modulated light. In this embodiment, the optical source 50 isan optical fiber that is driven by one or more light emitters, such aslaser diodes (not shown). The emitted beam 52 strikes a turning mirror54 and is directed toward a horizontal scanner 56. The horizontalscanner 56 is a mechanically resonant scanner that scans the beam 52periodically in a sinusoidal fashion. The horizontally scanned beam thentravels to a vertical scanner 58 that scans periodically to sweep thehorizontally scanned beam vertically. Eye coupling optics 60 then couplethe scanned beam 52 to an exit pupil expander 62 that provides anexpanded exit pupil for viewing by a viewer's eye 64. One such expanderis described in U.S. Pat. No. 5,701,132 of Kollin, et al., entitledVIRTUAL RETINAL DISPLAY WITH EXPANDED EXIT PUPIL, which is incorporatedherein by reference. One skilled in the art will recognize that, fordiffering applications, the exit pupil expander 62 may be omitted or mayhave a variety of structures, including diffractive or refractivedesigns. For example, the exit pupil expander 62 may be a planar orcurved structure and may create any number or pattern of output beams ina variety of patterns.

Returning to the description of scanning, as the beam scans through eachsuccessive location in a plane 66, the beam color and intensity ismodulated in a fashion to be described below to form a respective pixelof an image. By properly controlling the color and intensity of the beamfor each pixel location, the display 40 can produce the desired image.

The respective waveforms of the vertical and horizontal scanners areshown in FIGS. 4A and B respectively. In the plane 66 (FIG. 3), the beamtraces the pattern 68 shown in FIG. 5. As can be seen by comparing theactual scan pattern 68 to a desired raster scan pattern 69, the actualscanned beam 68 is "pinched" at the outer edges of the plane 66. Thatis, in successive forward and reverse sweeps of the beam, the pixelsnear the edge of the scan pattern are unevenly spaced. This unevenspacing can cause the pixels to overlap or can leave a gap betweenadjacent rows of pixels. Moreover, because image information istypically provided as an array of data, where each location in the arraycorresponds to a respective position in the ideal raster pattern 69, thedisplaced pixel locations can cause image distortion.

SUMMARY OF THE INVENTION

A display includes a primary scanning mechanism that simultaneouslyscans a beam of light both horizontally and vertically alongsubstantially continuous scan paths. To reduce raster pinch or tocorrect for certain types of distortion, the display also includes anauxiliary or correction scanner or other variable beam-shifting devicethat correctively redirects the beam.

In one embodiment, the scanning mechanism scans in a generally rasterpattern with a horizontal component and a vertical component. Amechanically resonant scanner produces the horizontal component byscanning the beam sinusoidally. A non-resonant or semi-resonant scannerscans the beam vertically along a generally linear scan path. Becausethe vertical scanner is moving during each sweep of the horizontalscanner, the vertical scanner imparts an initial vertical component tothe horizontal scan path. To reduce raster pinch due to the verticalcomponent, the auxiliary scanner adds a vertical component that offsetsthe initial vertical component.

In one embodiment the correction scanner operates at twice the frequencyof the horizontal scanner. The angular swing of the correction scanneris selected to equal the angular travel of the vertical scanner during ahorizontal sweep. For ease of fabrication, the correction scanner may bea resonant scanner having a resonant frequency at the desired correctionscan rate. In such embodiments, the auxiliary component of the scan doesnot precisely match the raster pinch; however, the resonant auxiliaryprovides a substantial improvement without a complicated scanningpattern.

Where the auxiliary scan frequency is twice the horizontal scanfrequency, the driving signal for the auxiliary scanner can be deriveddirectly from the horizontal scanner or the driving signal of horizontalscanner. In one embodiment, a position detector outputs an electricalsignal in response to a zero crossing or other repeated location in thehorizontal scan pattern. The electrical signal is filtered and amplifiedto produce a driving signal for the auxiliary scanner that is twice thehorizontal scan frequency.

In one embodiment, a displaced weight or other asymmetric feature isadded to the scanner so that the scanner resonates along or around adifferent axis from the primary scan axis. Where the additionalresonance is an integral multiple of the primary resonant frequency, theresulting scan pattern does not follow a straight line. For example, theresulting scan pattern can be a "bow tie" pattern where the off-axismovement offsets the motion of the vertical scan during horizontalsweeps.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagrammatic representation of a display aligned to aviewer's eye.

FIG. 2A is a combined image perceived by a user resulting from thecombination of light from an image source and light from a background.

FIG. 2B is an image perceived by a user from the display of FIG. 1 wherethe background light is occluded.

FIG. 3 is a diagrammatic representation of a scanner and a user's eyeshowing bidirectional scanning of a beam and coupling to the viewer'seye.

FIG. 4A is a signal-timing diagram of a vertical scanner in the scanningassembly of FIG. 1.

FIG. 4B is a signal-timing diagram of a drive signal for driving ahorizontal scanner in the scanning assembly of FIG. 1.

FIG. 5 is a signal position diagram showing the path followed by thescanned beam in response to the signals of FIGS. 4A and B.

FIG. 6 is a diagrammatic representation of a display according to theone embodiment invention.

FIG. 7 is an isometric view of a head-mounted scanner including atether.

FIG. 8 is a diagrammatic representation of a scanning assembly withinthe scanning display of FIG. 6, including a correction mirror.

FIG. 9 is an isometric view of a horizontal scanner and a verticalscanner suitable for use in the scanning assembly of FIG. 8.

FIG. 10 is a signal-timing diagram comparing a ramp signal with adesired signal for driving the vertical scanner.

FIG. 11 is a signal timing diagram showing positioning error andcorrection for the vertical scanning position.

FIG. 12 is a side cross sectional view of a piezoelectric correctionscanner.

FIG. 13A is a top plan view of a microelectromechanical (MEMs)correction scanner.

FIG. 13B is a side cross-sectional view of the MEMs correction scannerof FIG. 13A showing capacitive plates and their alignment to thescanning mirror.

FIG. 14 shows corrected scan position using a sinusoidally drivenscanner through 90% of the overall scan.

FIG. 15 shows an alternative embodiment of a reduced error scanner wherescan correction is realized by adding a vertical component to thehorizontal mirror.

FIG. 16 is a position diagram showing the scan path of a beam deflectedby the scanner of FIG. 15.

FIG. 17 is a diagrammatic view of a scanning system, including a biaxialmicroelectromechanical (MEMs) scanner and a MEMs correction scanner.

FIG. 18 is a diagrammatic view of a correction scanner that shifts aninput beam by shifting the position or angle of the input fiber.

FIG. 19 is a diagrammatic view of a correction scanner that includes anelectro-optic crystal that shifts the input beam in response to anelectrical signal.

FIG. 20 is a diagrammatic view of an imager that acquires external lightfrom a target object.

FIG. 21 is a diagrammatic view of an alternative embodiment of theimager of FIG. 20 that also projects a visible image.

FIG. 22 is a system block diagram showing handling of data to store datain a memory matrix while compensating for nonlinear scan speed of theresonant mirror.

FIG. 23 is a signal timing diagram showing deviation of a sinusoidalscan position versus time from the position of a linear scan.

FIG. 24 is a diagram showing diagrammatically how a linear set of countscan map to scan position for a sinusoidally scan.

FIG. 25 is a block diagram showing generation of an output clock toretrieve data from a memory matrix while compensating for nonlinear scanspeed of the resonant mirror.

FIG. 26 is a detail block diagram of a clock generation portion of theblock diagram of FIG. 25.

FIG. 27 is a block diagram of an alternative embodiment of the apparatusof FIG. 25 including pre-distortion.

FIG. 28 is a representation of a data structure showing datapredistorted to compensate for vertical optical distortion.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 6, a scanned beam display 70 according to oneembodiment of the invention is positioned for viewing by a viewer's eye72. The display 70 includes four principal portions, each of which willbe described in greater detail below. First, control electronics 74provide electrical signals that control operation of the display 70 inresponse to an image signal V_(IM) from an image source 76, such as acomputer, television receiver, videocassette player, or similar device.

The second portion of the display 70 is a light source 78 that outputs amodulated light beam 80 having a modulation corresponding to informationin the image signal V_(IM). The light source 78 may utilize coherentlight emitters, such as laser diodes or microlasers, or may usenon-coherent sources such as light emitting diodes. The light source 78may be a directly modulated light emitter such as a light emitting diode(LED) or may be include a continuous light emitter indirectly modulatedby an external modulator, such as an acousto-optic modulator.

The third portion of the display 70 is a scanning assembly 82 that scansthe modulated beam 80 of the light source 78 through a two-dimensionalscanning pattern, such as a raster pattern. The scanning assembly willbe described in greater detail below with reference to FIGS. 8-12.

Imaging optics 84 form the fourth portion of the display 70. The imagingoptics 84 in the embodiment of FIG. 6 include a pair of curved,partially transmissive mirrors 86 and 88 that shape and focus thescanned beam 80 appropriately for viewing by the eye 72. The scannedbeam 80 enters the eye 72 through a pupil 90 and strikes the retina 92.When scanned modulated light strikes the retina 92, the viewer perceivesthe image. The mirrors 86, 88 combine the light from the scanningassembly 82 with light received from a background 89 to produce acombined input to the viewer's eye 72. Although the background 89 ispresented herein as a "real-world" background, the background light maybe occluded or may be produced by another light source of the same ordifferent type.

Although the elements here are presented diagrammatically, one skilledin the art will recognize that the components are typically sized andconfigured for mounting to a helmet or similar frame as a head-mounteddisplay 70, as shown in FIG. 7. In this embodiment, a first portion 171of the display 70 is mounted to a head-borne frame 174 and a secondportion 176 is carried separately, for example in a hip belt. Theportions 174, 176 are linked by a fiber optic and electronic tether 178that carries optical and electronic signals from the second portion tothe first portion. An example of a fiber-coupled scanner display isfound in U.S. Pat. No. 5,596,339 of Furness et al. al., entitled VIRTUALRETINAL DISPLAY WITH FIBER OPTIC POINT SOURCE which is incorporatedherein by reference.

The scanning assembly 82 will be described next with reference to FIG.8. The scanning assembly 82 includes several components that correspondto the scanning source 42 of FIG. 3, where components common to thescanning assembly 82 and scanning source 42 are numbered the same.However, unlike the scanning source 42, the scanning assembly 82includes an active correction mirror 100 that can pivot to scan thelight beam 80 along the vertical axis. As will be explained below, thecorrection mirror 100 produces a varying corrective shift along thevertical axis during each sweep (forward or reverse) of the horizontalscanner 56. The corrective shift offsets vertical movement of the beam80 caused by the vertical scanner 58 to reduce the overall deviation ofthe scanning pattern from the desired pattern shown in broken lines inFIG. 5.

Before describing the effects of the correction mirror 100 and therelative timing of the various signals, exempting embodiments ofmechanically resonant scanners 200, 220 suitable for use as thehorizontal scanner 56 and vertical scanner 58 will be described withreference to FIG. 9.

The principal scanning component of the resonant scanner 200 is a movingmirror 202 mounted to a spring plate 204. The dimensions of the mirror202 and spring plate 204 and the material properties of the spring plate204 are selected so that the mirror 202 and spring plate 204 have a highQ with a natural oscillatory ("resonant") frequency on the order of1-100 kHz, where the selected resonant frequency depends upon theapplication. For VGA quality output with a 60 Hz refresh rate and nointerlacing, the resonant frequency is preferably about 15-20 kHz.

A ferromagnetic material mounted with the mirror 202 is driven by a pairof electromagnetic coils 206, 208 to provide motive force to mirror 202,thereby initiating and sustaining oscillation. The ferromagneticmaterial is preferably integral to the spring plate 204 and body of themirror 202. Drive electronics 218 provide electrical signals to activatethe coils 206, 208, as described above. Responsive to the electricalsignals, the coils 206, 208 produce periodic electromagnetic fields thatapply force to the ferromagnetic material, thereby causing oscillationof the mirror 202. If the frequency and phase of the electric signalsare properly synchronized with the movement of the mirror 202, themirror 202 oscillates at its resonant frequency with little powerconsumption.

The vertical scanner 220 is structured very similarly to the resonantscanner 200. Like the resonant scanner 201, the vertical scanner 220includes a mirror 222 driven by a pair of coils 224, 226 in response toelectrical signals from the drive electronics 218. However, because therate of oscillation is much lower for vertical scanning, the verticalscanner 220 is typically not resonant. The mirror 222 receives lightfrom the horizontal scanner 201 and produces vertical deflection atabout 30-100 Hz. Advantageously, the lower frequency allows the mirror222 to be significantly larger than the mirror 202, thereby reducingconstraints on the positioning of the vertical scanner 220. The detailsof virtual retinal displays and mechanical resonant scanning aredescribed in greater detail in U.S. Pat. No. 5,467,104, of Furness III,et al., entitled VIRTUAL RETINAL DISPLAY which is incorporated herein byreference.

One skilled in the art will recognize a variety of other structures thatmay scan a light beam through a generally raster pattern. For example, abidirectional microelectromechanical (MEMs) scanner may provide theprimary scanning. Such scanners are described in U.S. Pat. No. 5,629,790to Neukermanns et al. entitled MICROMACHINED TORSIONAL SCANNER, which isincorporated herein by reference. Like the scanning system describedabove, the horizontal components of the MEMs scanners are typicallydefined by mechanical resonances of their respective structures as isdescribed in greater detail below with reference to FIG. 16. Like thetwo scanner system described above with reference to FIG. 3, thesebiaxial scanners typically suffer similar raster pinch problems due tomovement along the slower scan axis during sweeps along the faster scanaxis.

The light source 78 outputs a beam of light that is modulated accordingto the image signal from the drive electronics 218. At the same time,the drive electronics 218 activate the coils 206, 208, 224, 226 tooscillate the mirrors 202, 222. The modulated beam of light strikes theoscillating horizontal mirror 202, and is deflected horizontally by anangle corresponding to the instantaneous angle of the mirror 202. Thedeflected light then strikes the vertical mirror 222 and is deflected ata vertical angle corresponding to the instantaneous angle of thevertical mirror 222. As will also be described below, the modulation ofthe optical beam is synchronized with the horizontal and vertical scansso that at each position of the mirrors, the beam color and intensitycorrespond to a desired virtual image. The beam therefore "draws" thevirtual image directly upon the user's retina.

One skilled in the art will recognize that several components of thescanner 200 have been omitted from the FIG. 9 for clarity ofpresentation. For example, the horizontal and vertical scanners 201, 220are typically mounted in fixed relative positions to a frame.Additionally, the scanner 200 typically includes one or more turningmirrors that direct the beam such that the beam strikes each of themirrors a plurality of times to increase the angular range of scanning.

Returning to FIG. 8, the operation of the system, including thecorrection mirror 100 will now be described. For purposes of clarity forthe following discussion, it will be assumed that, at the "zero"positions of the mirrors 100, 56, 58 (i.e., the mirrors are centered),the beam 80 is centered in the plane 66. One skilled in the art willrecognize that the zero position can be selected arbitrarily in mostcases with straightforward adaptations of the angles and paths describedbelow.

As can be seen by ray tracing, the position of the beam 80 in the plane66 will be a function of the angular deflections from the turning mirror100, the horizontal scanner 56, and the vertical scanner 58. The actualvector angle of the beam 80 at any point in time can then be determinedby vector addition. In most cases, the desired vertical portion of thescan pattern will be a "stair step" scan pattern, as shown by the brokenline in FIG. 10.

If the turning mirror 100 is disabled, the pattern traced by the raywill be the same as that described above with respect to FIGS. 3-5. Asshown in FIG. 10, the actual vertical scan portion of the pattern, shownin solid line, will be an approximate ramp, rather than the desiredstair step pattern.

One approach to providing the stair step pattern would be to drive thevertical scanner 58 with a stair step voltage. However, because thevertical mirror is a physical system and the stair step involvesdiscontinuous motion, the vertical mirror will not follow the drivesignal exactly. Instead, as the vertical mirror attempts to follow thestair step pattern, the vertical mirror will move at a maximum ratedictated largely by the size and weight of the vertical mirror, thematerial properties of the mirror support structure, the peak voltage orcurrent of the driving signal, and electrical properties of the drivingcircuitry. For typical vertical scan mirror size, configuration, scanangle and driving voltage, the vertical scanner 58 is limited tofrequencies on the order of 100 to 3000 Hz. The desired scan pattern hasfrequency components far exceeding this range. Consequently, driving thevertical scanner 58 with a stair step driving signal produces a verticalscan pattern that deviates significantly from the desired pattern.

To reduce this problem, the embodiment of FIG. 8 separates the verticalscan function into two parts. The overall vertical scan is then acombination of a large amplitude ramp function at about 60 Hz and asmall amplitude correction function at twice the horizontal rate (e.g.,about 30 kHz). The vertical scanner 58 can produce the large amplituderamp function easily, because the 60 Hz frequency is well below theresponse frequency of typical scanning mirrors. The correction mirror100 operates at a much higher frequency; however, the overall angularswing of the correction mirror 100 is very small.

As can be seen from the signal timing diagrams of FIG. 10, thecorrection mirror 100 travels from approximately its maximum negativeangle to its maximum positive angle during the time that the horizontalscanner scans from the one edge of the field of view to the oppositeedge (i.e. from time t₁ to t₂ in FIG. 11). The overall correction angle,as shown in FIG. 5, is defined by the amount of downward travel of thevertical scan mirror during a single horizontal scan. The correctionangle will vary for various configurations of the display; however, thecorrection angle can be calculated easily.

For example, for a display having 1280 vertical lines, and a totalmechanical vertical scan angle of 10 degrees, the angular scan range foreach line is about 0.008 degrees (10/1280=0.0078125). Assuming thevertical scanner travels this entire distance during the horizontal scanan error correction to be supplied by the correction mirror 100 of aboutplus or minus 0.0039 degrees. The angular correction is thusapproximately θ/N, where θ is the vertical scan angle and N is thenumber of horizontal lines. This number may be modified in someembodiments. For example, where the horizontal scanner is a resonantscanner, the correction angle may be slightly different, because thehorizontal scanner will use some portion of the scan time to halt andbegin travel in the reverse direction, as the scan reaches the edge ofthe field of view.

As can be seen from the timing diagrams of FIGS. 5 and 10, thecorrection mirror 100 will translate the beam vertically by about onehalf of one line width at a frequency of twice that of the horizontalscanner 56. For a typical display at SVGA image quality, the horizontalscanner 56 will resonate at about 15 kHz. Thus, for a typical display,the correction scanner 100 will pivot by about one-half of one degree atabout 30 kHz. One skilled in the art will recognize that, as theresolution of the display increases, the scan rate of the horizontalscanner 56 increases. The scan rate of the correction mirror 100 willincrease accordingly; but, the pivot angle will decrease. For example,for a display having 2560 lines and an overall scan of 10 degrees, thescan rate of the correction mirror 100 will be about 60 kHz with a pivotangle of about 0.002 degrees.

FIG. 12 shows a piezoelectric scanner 110 suitable for the correctionmirror 100 in some embodiments. The scanner 110 is formed from aplatform 112 that carries a pair of spaced-apart piezoelectric actuators114, 116. The correction mirror 100 is a metallized, substantiallyplanar silicon substrate that extends between the actuators 114, 116.The opposite sides of the piezoelectric actuators 114, 116 areconductively coated and coupled to a drive amplifier 120 such that thevoltages across the actuators 114, 116 are opposite. As is known,piezoelectric materials deform in the presence of electric fields.Consequently, when the drive amplifier 120 outputs a voltage, theactuators 114, 116 apply forces in opposite directions to the correctionmirror 100, thereby causing the correction mirror 100 to pivot. Oneskilled in the art will recognize that, although the piezoelectricactuators 114, 116 are presented as having a single set of electrodesand a single layer of piezoelectric material, the actuators 114, 116would typically be formed from several layers. Such structures are usedin commercially available piezoelectric devices to produce relativelylarge deformations.

A signal generator circuit 122 provides the driving signal for the driveamplifier 120 in response to the detected position of the horizontalscanner 56. The principal input to the circuit 122 is a sense signalfrom a sensor coupled to the horizontal scanner 56. The sense signal canbe obtained in a variety of approaches. For example, as described inU.S. Pat. No. 5,648,618 to Neukermanns et al., entitled MICROMACHINEDHINGE HAVING AN INTEGRAL TORSIONAL SENSOR, which is incorporated hereinby reference, torsional movement of a MEMs scanner can produceelectrical outputs corresponding to the position of the scanning mirror.Alternatively, the position of the mirror may be obtained by mountingpiezoelectric sensors to the scanner, as described in U.S. Pat. No.5,694,237 to Melville, entitled POSITION DETECTION OF MECHANICALRESONANT SCANNER MIRROR, which is incorporated herein reference. Inother alternatives, the position of the beam can be determined byoptically or electrically monitoring the position of the horizontal orvertical mirrors or by monitoring current induced in the mirror drivecoils.

When the sense signal indicates that the horizontal scanner 56 is at theedge of the field of view, the circuit 122 generates a ramp signal thatbegins at its negative maximum and reaches its zero crossing point whenthe horizontal scanner reaches the middle of the field of view. The rampsignal then reaches its maximum value when the horizontal scan reachesthe opposite edge of the field of view. The ramp signal returns to itsnegative maximum during the interval when the horizontal scan slows to ahalt and begins a return sweep. Because the circuit 122 can use thesense signal as the basic clock signal for the ramp signal, timing ofthe ramp signal is inherently synchronized to the horizontal position ofthe scan. However, one skilled in the art will recognize that, for someembodiments a controlled phase shift of the ramp signal relative to thesense signal will optimize performance. One skilled in the art will alsorecognize that where the correction mirror is scanned resonantly, asdescribed below with reference to FIG. 16, the ramp signal can bereplaced by a sinusoidal signal, that can be obtained simply byfrequency doubling, amplifying and phase shifting the sense signal.

The vertical movement of the beam induced by the correction mirror 100offsets the movement of the beam caused by the vertical scanner 58, sothat the beam remains stationary along the vertical axis during thehorizontal scan. During the time the horizontal scan is out of the fieldof view, beam travels vertically in response to the correction mirror100 to the nominal position of the next horizontal scan.

As can be seen from the above discussion, the addition of thepiezoelectrically driven correction mirror 100 can reduce the rasterpinching significantly with a ramp-type of motion. However, in someapplications, it may be undesirable to utilize ramp-type motion. Onealternative embodiment of a scanner 130 that can be used for thecorrection mirror 100 is shown in FIGS. 13A and 13B.

The scanner 130 is a resonant microelectromechanical (MEMs) scanner,fabricated similarly to those described in the Neukermans '790 patent,except that processing is simplified because the scanner 130 isuniaxial. Alternatively, the scanner 130 can be a mechanically resonantscanner very similar to the horizontal scanner 54 of FIG. 9; however, insuch a scanner it is preferred that the dimensions and materialproperties of the plate and mirror be selected to produce resonance atabout 30 kHz, which is twice the resonant frequency of the horizontalscanner 200. Further, the materials and mounting are preferably selectedso that the scanner 130 has a much lower Q than the Q of the horizontalscanner 56. The lower Q allows scanner 130 to operate over a broaderrange of frequencies, so that the scanner 130 can be tuned to anintegral multiple of the horizontal scan frequency.

The use of the resonant scanner 130 can reduce the complexity of theelectrical components for driving the scanner 130. However, because thescanner 130 is resonant, it will tend to have a sinusoidal motion,rather than the ramp-type motion described above. However, if thefrequency, phase, and amplitude of the sinusoidal motion are selectedappropriately, the correction mirror 100 can reduce the pinch errorsignificantly. For example, FIG. 14 shows correction of the rastersignal with a sinusoidal motion of the correction mirror where thehorizontal field of view encompasses 90 percent of the overallhorizontal scan angle. One skilled in the art will recognize that theerror in position of the beam can be reduced further if the field ofview is a smaller percentage of the overall horizontal scan angle.Moreover, even further reductions in the scan error can be realized byadding a second correction mirror in the beam path, although this isgenerally undesirable due to the limited improvement versus cost.Another approach to reducing the error is to add one or more higherorder harmonics to the scanner drive signal so that the scanning patternshifts from a sinusoidal scan closer to a triangle wave.

Another alternative embodiment of a reduced error scanner 140 is shownin FIG. 15 where the scan correction is realized by adding a verticalcomponent to a horizontal mirror 141. In this embodiment, the horizontalscanner 140 is a MEMs scanner having an electrostatic drive to pivot thescan mirror. The horizontal scanner 140 includes an army of locations143 at which small masses 145 may be formed. The masses 145 may bedeposited metal or other material that is formed in a conventionalmanner, such as photolithography. The masses 143 are locatedasymmetrically about a centerline 147 of the mirror 141. The masses 145provide a component to scan the correction along the vertical axis bypivoting about an axis orthogonal to its primary axis; as can be seen inFIG. 16, the vertical scan frequency is double the horizontal scanfrequency, thereby producing a Lissajous or "bow-tie" overall scanpattern. The masses 145 may be actively varied (e.g. by laser ablation)to tune the resonant frequency of the vertical component. Thisembodiment allows correction without an additional mirror, but typicallyrequires matching the resonant frequencies of the vibration and thehorizontal scanner.

As shown in FIG. 17, another embodiment of a scanner 150 according tothe invention employs a biaxial scanner 152 as the principal scancomponent, along with a correction scanner 154. The biaxial scanner 152is a single mirror device that oscillates about two orthogonal axes.Design, fabrication and operation of such scanners are described forexample in the Neukermans '790 patent and in Kiang, et al.,MICROMACHINED MICROSCANNERS FOR OPTICAL SCANNING, SPIE Proceedings onMiniaturized Systems with Micro-Optics and Micromachines II, Vol. 3008,pp. 82-90 which is incorporated herein by reference.

The correction scanner 154 is preferably a MEMs scanner, although othertypes of scanners, such as piezoelectric scanners may also be within thescope of the invention. As described above, the correction mirror 154can scan sinusoidally to remove a significant portion of the scan error;or, the correction mirror can scan in a ramp pattern for more preciseerror correction.

Light from the light source 78 strikes the correction mirror 154 and isdeflected by a correction angle as described above. The light thenstrikes the biaxial scanner 152 and is scanned horizontally andvertically to approximate a raster pattern, as described above withreference to FIGS. 3-5. As described above, the overall pattern moreclosely approximates a raster pattern.

Another embodiment of a display according to the invention, shown inFIG. 18, eliminates the correction mirror 100 by physically shifting theinput beam laterally relative to the input of an optical system 500. Inthe embodiment of FIG. 18, a piezoelectric driver 502 positioned betweena frame 504 and an input fiber 506 receives a drive voltage at afrequency twice that of the horizontal scan frequency. Responsive to thedrive voltage, the piezoelectric driver 502 deforms. Because the fiber506 is bonded to the piezoelectric driver 502, deformation of thepiezoelectric driver 502 produces corresponding shifting of the fiber506 as indicated by the arrow 508 and shadowed fiber 510. One skilled inthe art will recognize that, depending upon the characteristics of theoptical system 500, the piezoelectric driver 502 may produce lateraltranslation of the fiber 506 or angular shifting of the fiber 506output. The optical system 500 then translates movement of the fiberoutput into movement of the perceived pixel location as in thepreviously described embodiments. While the embodiment of FIG. 18utilizes translating a fiber, the invention is not so limited. Forexample some applications may incorporate translation of other sources,such as LEDs or laser diodes.

Although the embodiment of FIG. 18 shifts the input beam by shifting theposition or angle of the input fiber other methods of shifting the inputbeam may be within the scope of the invention. For example, as shown inFIG. 19, an electro-optic crystal 300 shifts the input beam 83 inresponse to an electrical signal. In this embodiment, the beam 83 entersa first face 302 of a trapezoidally shaped electro-optic crystal 300,where refraction causes a shift in the direction of propagation. Whenthe beam 83 exits through a second face 304, refraction produces asecond shift in the direction of propagation. At each face, the amountof changes in the direction of propagation will depend upon differencein index of refraction between the air and the crystal 300. As is known,the index of refraction of electro-optic crystals is dependent upon theelectric field through the crystal. A voltage applied across the crystal300 through a pair of electrodes 306 can control the index of refractionof the crystal. Thus, the applied voltage can control the angular shiftof the beam 83 as it enters and exits the crystal 300 as indicated bythe broken line 83A. The amount of shift will correspond to the appliedvoltage. Accordingly, the amount of shift can be controlled bycontrolling the voltage applied to the electrodes 306. The crystal 300thus provides a voltage controlled beam shifter that can offset rasterpinch.

Although the embodiments described herein have been displays, otherdevices or methods may be within the scope of the invention. Forexample, as shown in FIG. 20, an imager 600 includes a biaxial scanner602 and correction scanner 604 that are very similar to the scanners152, 154 of FIG. 17. The imager 600 is an image collecting device thatmay be the input element of a digital camera, bar code reader, or otherimage acquisition device. To allow the imager 600 to gather lightefficiently, the imager 600 includes gathering optics 606 that collectand transmit light from a target object 608 outside of the imager 600onto the correction scanner 604. The gathering optics 606 are configuredto have a depth of field, focal length, field of view and other opticalcharacteristics appropriate for the particular application. For example,where the imager 600 is a two dimensional symbology reader, thegathering optics may be optimized for red or infrared light and thefocal length may be in the order of 10-50 cm.

The correction scanner 604 redirects light received from the gatheringoptics 606 as described above for the display embodiments, so that thegathered light has a correction component before it reaches the biaxialscanner 602. The biaxial scanner scans through a substantially rasterpattern to collect light arriving at the gathering optics from a rangeof angles and to redirect the light onto a stationary photodetector 610.Movement of the biaxial scanner 602 thus translates to imagingsuccessive points of the target object 608 onto the photodetector 610.The photodetector 610 converts light energy from the scanner 602 intoelectrical signals that are received by decoding electronics 612. Wherethe imager 600 is a symbology reader, the decoding electronics 612 mayinclude symbol decoding and storing circuitry. Where the imager is aportion of a camera, the decoding electronics 612 may include adigital-to-analog converter, a memory device and associated electronicsfor storing a digital representation of the scanned target object 608.

Another feature of the imager 600 shown in FIG. 20 is an illuminationsource 614 that provides light for illuminating a target object. Theillumination source 614 may be one of many types, depending upon theapplication. For example, where the imager 600 is a symbol reader, theillumination source 614 may be an infrared or red light emitter thatemits a beam of light into a beam splitter 616. The beam splitter 616directs the illuminating light beam onto the biaxial scanner 602 wherethe illuminating light is redirected to the correction scanner 604.Because the illuminating light is collinear with the path of light fromthe target object 608, the illuminating light strikes the target object608 at the same location that is imaged by the photodetector 610. Theilluminating light is reflected by the target object 608 in a patterncorresponding to the reflectivity of the target object 608. Thereflected illuminating light travels to the photodetecor 610 andprovides light that can be used by the photodetector 610 to image thetarget object 608.

In one application of the imager 600 of FIG. 20, the illumination source614 is a visible, directly modulatable light source, such as a red laserdiode or a visible wavelength light emitting diode (LED). As shown inFIG. 21, the illumination source 614 can thus produce a visible imagefor the user. In the exemplary embodiment of FIG. 21, the imager canoperate as a symbology scanner to identify information contained in asymbol on the target object 608. Once the decoding electronics 612identifies the information represented by the symbol, the decodingelectronics 612 identifies a desired image to be viewed, such as an itemprice and identity. The decoding electronics 612 modulates the drivecurrent of the illumination source 614 to modulate the intensity of theemitted light according to the desired image. When the user directs theimager 600 toward a screen 616, the illumining light is scanned onto thescreen 616 as described above. Because the illuminating light ismodulated according the desired image, the light reflected from thescreen 616 is spatially modulated according to the desired image. Theimager 600 thus acts as an image projector in addition to acquiringimage data.

In addition to compensating for raster pinch, one embodiment of thescanning system, shown in FIG. 22, also addresses effects of thenonlinearity of resonant and other nonlinear scanning systems. As shownby broken line in FIG. 23, the timing of incoming data is premised upona linear scan rate. That is, for equally spaced subsequent locations ina line, the data arrive at constant intervals. A resonant scanner,however, has a scan rate that varies sinusoidally, as indicated by thesolid line. For a start of line beginning at time to (note that theactual start of scan for a sinusoidal scan would likely be delayedslightly as described above with respect to FIG. 14), the sinusoidalscan initially lags the linear scan. Thus, if image data for position P₁arrive at time t_(1A), the sinusoidal scan will place the pixel atposition P₂.

To place the pixel correctly, the system of FIG. 22 delays the imagedata until time t_(1B), as will now be described with reference to FIGS.22 and 24. Arriving image data V_(IM) are clocked into a line or framebuffer 2200 by a counter circuit 2202 in response to a horizontalsynchronization component of the image data signal. The counter circuit2202 is a conventional type circuit, and provides an input clock signalhaving equally spaced pulses to clock the data into the buffer 2200.

A feedback circuit 2204 controls timing of output from the buffer 2200.The feedback circuit 2204 receives a sinusoidal or other sense signalfrom the scanning assembly 82 and divides the period of the sense signalwith a high speed second counter 2206. A logic circuit 2208 produces anoutput clock signal in response to the counter output

Unlike the input clock signal, however, pulses of the output clocksignal are not equally spaced. Instead, the pulse timing is determinedanalytically by comparing the timing of the linear signal of FIG. 23 tothe sinusoidal signal. For example, for a pixel to be located atposition P₁, the logic circuit 2208 provides an output pulse at timet_(1B), rather that time t_(1A), as would be the case for a linear scanrate.

The logic circuit 2208 identifies the count corresponding to a pixellocation by accessing a look-up table in a memory 2210. Data in thelook-up table are defined by dividing the scanning system period intomany counts and identifying the count corresponding to the proper pixellocation. FIG. 24 shows this evaluation graphically for a 35-pixel line.One skilled in the art will recognize that this example is simplifiedfor clarity of presentation. A typical line may include hundreds or eventhousands of pixels. As can be seen, the pixels will be spacedundesirably close at the edges of the field of view and undesirably farat the center of the field of view. Consequently, the image will becompressed near the edges of the field of view and expanded near themiddle, forming a distorted image.

As shown by the upper line, pixel location varies nonlinearly for pixelcounts equally spaced in time. Accordingly, the desired locations ofeach of the pixels, shown by the lower line, actually correspond tononlinearly spaced counts. For example, the first pixel in the upper andlower lines arrives at the zero count and should be located in the zerocount location. The second pixel arrives at the 100 count, but, shouldbe positioned at the 540 count location. Similarly, the third pixelarrives at count 200 and is output at count 720. One skilled in the artwill recognize that the figure is merely representative of the actualcalculation and timing. For example, some output counts will be higherthan their corresponding input counts and some counts will be lower. Ofcourse, a pixel will not actually be output before its correspondingdata arrives. To address this condition, the system of FIG. 22 actuallyimposes a latency on the output of data, in a similar fashion tosynchronous memory devices. For the example of FIG. 24, a single linelatency (3400 count latency) would be ample. With such a latency, thefirst output pixel would occur at count 3400 and the second would occurat count 3940.

FIG. 25 shows an alternative approach to placing the pixels in theproper locations. This embodiment produces a corrected clock from apattern generator rather that a counter to control clocking of outputdata. A synch signal stripper 2500 strips the horizontal synchronizationsignal from an arriving image signal V_(IM). Responsive to the synchsignal, a phase locked loop 2502 produces a series of clock pulses thatare locked to the synch signal. An A/D converter 2504, driven by theclock pulses, samples the video portion of the image signal to producesampled input data. The sampling rate will depend upon the requiredresolution of the system. In the preferred embodiment, the sampling rateis approximately 40 Mhz. A programmable gate array 2506 conditions thedata from the A/D converter 2504 to produce a set of image data that arestored in a buffer 2508. One skilled in the art will recognize that, foreach horizontal synch signal, the buffer will receive one line of imagedata. For a 1480×1024 pixel display, The system will sample and store1480 sets of image data during a single period of the video signal.

Once each line of data is stored in the buffer 2508, the buffer isclocked to output the data to a RAMDAC 2509 that includes a gammacorrection memory 2510 containing corrected data. Instead of using thebuffer data as a data input to the gamma correction memory 2510, thebuffer data is used to produce addressing data to retrieve the correcteddata from the gamma correction memory 2510. For example, for a set ofimage data corresponding to a selected image intensity I1 identifies acorresponding location in the gamma correction memory 2510. Rather thanoutput the actual image data, the gamma correction memory 2510 outputs aset of corrected data that will produce the proper light intensity atthe user's eye. The corrected data is determined analytically andempirically by characterizing the overall scanning system, including thetransmissivity of various components, the intensity versus currentresponse of the light source, diffractive and aperture effects of thecomponents and a variety of other system characteristics.

The corrected data output from the gamma correction memory 2510 drives aD/A converter 2512 to produce a gamma corrected analog signal. A scannerdrive circuit 2514 amplifies and processes the corrected analog signalto produce an input signal to a light source 2516. In response the lightsource 2516 outputs light modulated according to the corrected data fromthe gamma correction memory 2510. The modulated light enters a scanner2518 to produce scanned, modulated light for viewing.

The clock signal that drives the buffer 2508, correction memory 2510,and D/A converter 2512 comes from a corrected clock circuit 2520 thatincludes a clock generator 2522, pattern memory 2524 and rising edgedetector 2526. The clock generator 2522 includes a phase locked loop(PLL) that is locked to a sense signal from the scanner 2518. The PLLgenerates a high frequency clock signal at about 80 MHz that is lockedto the sense signal. The high frequency clock signal clocks datasequentially from addresses in the pattern memory 2524.

The rising edge detector 2526 outputs a pulse in response to eachtransition 0-to-1 transition of the data retrieved from the patternmemory 2524. The pulses then form the clock signal that drives thebuffer output, gamma correction memory 2510, and D/A converter 2512.

One skilled in the art will recognize that the timing of pulses outputfrom the edge detector 2526 will depend upon the data stored in thepattern memory 2524 and upon the scanning frequency f_(SCAN) of thescanner 2518. FIG. 26 shows a simplified example of the concept. Oneskilled in the art will recognize that, in FIG. 26, the data structureis simplified and addressing and other circuitry have also been omittedfor clarity of presentation.

In the example, if the scanning frequency f_(SCAN) is 20 kHz and clockgenerator 2522 outputs a clock signal at 4000 times the scanningfrequency f_(SCAN), the pattern memory 2524 is clocked at 80 MHz. If allbits in an addressed memory location 2524A are 0, no transitions of theoutput clock occur for 16 transitions of the generator clock. For thedata structure of location 2524B, a single transition of the outputclock occurs for 16 transitions of the generator clock. The number andrelative timing of the pulses is thus controlled by the data stored inthe pattern memory 2524. The frequency of the generator clock on theother hand depends upon the scanner frequency. As the scanner frequencyvaries, the timing of he pulses thus will vary.

The approach of FIG. 25 is not limited to sinusoidal rate variationcorrection. The clock pattern memory 2524 can be programmed to addressmany other kinds of nonlinear effects, such as optical distortion,secondary harmonics, and response time idiosyncrasies of the electronicsor optical source.

Moreover, the basic structure of FIG. 25 can be modified easily, byinserting a bit counter 2530, look up table 2532, and verticalincrementing circuit 2534 and as shown in FIG. 27. The counter 2530addresses the look up table in response to each pulse of the input clockto retrieve two bits of stored data. The retrieved data indicate whetherthe vertical address should be incremented, decremented or leftunaffected. If the address is to be incremented or decremented, theincrementing circuit increments or decrements the address in the buffer2508, so that data that were to be stored in a nominal memory locationare actually stored in an alternate location that is one row higher orlower than the nominial location.

A graphical representation of one such data structure is shown in thesimplified example FIG. 28. In this example, the first three sets ofdata bits for the first line of data (line 0) are stored in the firstmemory row, the next three sets of data bits for the first line arestored in the second memory row, and the last three sets of data bitsfor the first line are stored in the third memory row. One skilled inthe art will recognize that this example has been greatly simplified forclarity of presentation. An actual implementation would include manymore sets of data.

The result is that some portion of the data for one line is moved to anew line. The resulting data map in the buffer 2508 is thus distorted ascan be seen from FIG. 28. However, distortion of the data map can beselected to offset vertical distortion of the image caused by scanningand optical distortion. The result is that the overall system distortionis reduced.

Although the invention has been described herein by way of exemplaryembodiments, variations in the structures and methods described hereinmay be made without departing from the spirit and scope of theinvention. For example, the positioning of the various components mayalso be varied. In one example of repositioning, the correction scannercan be positioned in the optical path either before or after the otherscanners. Also, the exit pupil expander may be omitted in manyapplications. In such embodiments, conventional eye tracking may beadded to ease coupling of the scanned beam to the eye. Moreover, thescanning system can be used for projection displays, optical storage anda variety of other scanned light beam applications. Further, a varietyof other timing control mechanisms, such as programmable delays, may beused to compensate for the variable speed of the scanner in place of theapproaches described with reference to FIGS. 22-28. In anotheralternative approach to timing and distortion correction, the memory mapmay be undistorted and addressed at a constant rate. In such anapproach, the data are output from the buffer 2508 at a constant rate.To compensate for nonlinearity of the scanner, the data for eachlocation are derived from the retrieved image data and output at a fixedincrements. Referring to FIG. 24, for example, data would be output attime 1500, even though this time did not correspond directly to a pixeltime. To compensate, the buffer 2508 is addressed at the 10th and 11thlocations for this line. Then, the output data is a weighted average ofthe data from the 10th and 11th locations. Thus, the buffer 2508 isclocked at a constant rate and pixels are output at a constant rate.Yet, by controlig the addressing circuitry carefully and performing aweighted averaging, the output data is sinusoidally corrected.Accordingly, the invention is not limited except as by the appendedclaims.

What is claimed is:
 1. A method of producing an image for viewing,comprising the steps of:emitting light from a first location; resonantlyscanning the light along a first axis at a first frequency; scanning thelight along a second axis different from the first axis at a secondfrequency, while scanning the light along the first axis; scanning thelight along the second axis at a third frequency that is an integralmultiple of the first frequency, while scanning the light along thefirst axis; and modulating the light in a pattern corresponding to theimage, synchronously with the step of resonantly scanning the lightalong the second axis.
 2. The method of claim 1 wherein the step ofscanning the light along the second axis at a third frequency includesresonantly scanning at the third frequency.
 3. The method of claim 1wherein the step of scanning the light along the second axis at a thirdfrequency includes the steps of:scanning a turning mirror with apiezoelectric scanner at the third frequency; and redirecting the lightwith the scanned turning mirror.
 4. The method of claim 1 wherein thestep of scanning the light along the second axis at a third frequencythat is an integral multiple of the first frequency, while scanning thelight along the first axis includes the steps of:sensing a scanningposition of the light along the first axis; producing a driving signalin response to the sensed scanning position; and scanning the lightalong the second axis in response to the produced driving signal.
 5. Themethod of claim 4 wherein the step of producing a driving signal inresponse to the sensed scanning position includes the steps of:producinga sense signal corresponding to the sensed scanning position; andfrequency doubling the sense signal.
 6. A method of scanning a lightbeam in a substantially raster pattern, comprising the stepsof:emitting, from a first position, the light beam; scanning the lightbeam about a first axis through a first angular range at a first ratewith a first period; scanning the light beam about a second axisorthogonal to the first axis through a second angular range at a secondrate; directing the emitted, scanned light toward the user's eye; andscanning the light beam at a third rate at least as high as the firstrate about the second axis at an amplitude selected to offset motion ofthe second scan during the first period.
 7. The method of claim 6wherein the third rate is twice the first rate.
 8. The method of claim 6wherein the steps of scanning the light beam about a first axis througha first angular range at a first rate with a first period and scanningthe light beam about a second axis orthogonal to the first axis througha second angular range at a second rate, include sweeping a mirror aboutboth the first and second axes.
 9. The method of claim 6 wherein thestep of scanning the light beam at a third rate at least as high as thefirst rate along the second axis at an amplitude selected to offsetmotion of the second scan during the first period includes the stepsof:determining the position of the beam about the first axis; producingan electrical signal indicative of the determined position; generating adrive signal in response to the electrical signal; and driving a scannerwith the drive signal to scan the light at the third rate.
 10. Themethod of claim 9 wherein the step of generating a driving signalincludes the step of frequency doubling the electrical signal indicativeof the position of the beam about the first axis.
 11. A method ofscanning an optical path through a substantially rectilinear pattern,comprising the steps of:scanning a first mirror periodically in a firstdirection at a first frequency, the first mirror being positioned tosweep the optical path about a first axis; scanning a second mirrorcontinuously in a second direction while scanning the first mirror inthe first direction, the second mirror being positioned to sweep theoptical path about a second axis different from the first axis;producing a scanning signal at a second frequency that is twice thefirst frequency of the first frequency; and scanning a third mirror inresponse to the scanning signal, the third mirror being positioned tosweep the optical path about the second axis.
 12. The method of claim 11wherein the first and second mirrors are the same mirror.
 13. The methodof claim 11 wherein the first and second mirrors are different mirrors.14. The method of claim 11 wherein the step of scanning a first mirrorperiodically in a first direction at a first frequency, includesactivating a resonant scanner.
 15. The method of claim 11 wherein thestep of scanning a third mirror in response to the scanning signal,includes activating a resonant correction scanner having a resonantfrequency at the frequency of the scanning signal.
 16. The method ofclaim 15 further including varying the resonant frequency of theresonant correction scanner.
 17. A method of scanning an optical paththrough a periodic pattern with a scanning system including amechanically resonant scanner having a resonant frequency, comprisingthe steps of:scanning the optical path through a field of view at theresonant frequency along a first axis by activating the mechanicallyresonant scanner; scanning, at a frequency lower than the resonantfrequency, the optical path along a second axis different from the firstaxis while performing the step of scanning the optical path along thefirst axis by activating the mechanically resonant scanner; determiningan the amount of scan of the optical path along the second axis thatoccurs while the optical path scans once through the field of view;producing a driving signal at a correction frequency that is an integralmultiple of the resonant frequency; and scanning along the second axisat the correction frequency and with an amplitude selected to offset thedetermined amount of scan.
 18. The method of claim 17 wherein the stepof scanning along the second axis at the correction frequency and withan amplitude selected to offset the determined amount of scan, includesactivating a resonant correction scanner having a resonant frequency atthe correction frequency.
 19. The method of claim 15 further includingvarying the resonant frequency of the correction scanner.
 20. A scannerfor scanning a beam of electromagnetic energy through a substantiallyraster pattern, comprising:a first scanning assembly having a firstmirror configured to pivot about a first axis and a second mirrorconfigured to pivot about a second axis orthogonal to the first axis; asecond scanning assembly having a third mirror separate from the firstmirror and the second mirror, the third mirror being pivotable about thefirst axis in response to a driving signal; a position sensor having asensing input coupled to the first mirror and a sensing output, theposition sensor being responsive to movement of the first mirror aboutthe first axis to produce an electrical signal at the sensing outputcorresponding to the position of the first mirror; and a driving circuithaving a signal input coupled to the sensing output and a driving outputcoupled to the second scanning assembly, the driving circuit beingresponsive to the electrical signal to produce the driving signal. 21.The scanner of claim 20 wherein the first and second mirrors are thesame mirror.
 22. The scanner of claim 20 wherein the first scanningassembly is a resonant assembly having a first resonant frequency. 23.The scanner of claim 22 wherein the third scanning assembly is aresonant assembly having a third resonant frequency.
 24. The scanner ofclaim 23 wherein the third resonant frequency is twice the firstresonant frequency.
 25. The scanner of claim 24 wherein the firstscanning assembly includes a first MEMs scanner.
 26. The scanner ofclaim 25 wherein the third scanning assembly includes a third MEMsscanner.
 27. The scanner of claim 25 wherein the first MEMs scanner isbiaxial.
 28. A scanning apparatus for scanning a beam in a substantiallyraster format, comprising:a first scanning assembly having a firstoptical input and a first scan signal input, the first scanning assemblybeing configured to scan an optical beam substantially sinusoidally at afirst frequency about a first axis and to scan the optical beam about asecond axis orthogonal to the first axis; and a corrective scannerpositioned to receive the optical beam either before or after the firstscanning assembly and configured to scan the beam about the second axisat a second frequency that is twice the first frequency.
 29. Thescanning apparatus of claim 28 wherein the corrective scanner has anangular range equal to an expected angle of travel of the first scanningassembly about the second axis during a single scan of the firstscanning assembly about the first axis.
 30. The scanning apparatus ofclaim 29 wherein the first scanning assembly includes a first reflectivesurface that pivots through a first angular range about the first axis.31. The scanning apparatus of claim 30 wherein the first reflectivesurface pivots through a second angular range about the second axis. 32.The scanning apparatus of claim 30 wherein the first scanning assemblyincludes a second reflective surface that pivots through a secondangular range about the second axis.
 33. The scanning apparatus of claim29 wherein the first scanning assembly has a resonant mode at the firstfrequency.
 34. The scanning apparatus of claim 29 wherein the correctionscanner has a resonant mode at twice the first frequency.
 35. Thescanning apparatus of claim 30 wherein the first scanning assembly is aMEMs scanner.
 36. The scanning apparatus of claim 35 wherein the MEMsscanner is a biaxial scanner.
 37. The scanning apparatus of claim 36wherein the MEMs scanner is a resonant scanner.
 38. The scanningapparatus of claim 30 wherein the first scanning assembly includes asensor responsive to provide a sense signal indicative of the angle ofthe optical beam about the first axis.
 39. The scanning apparatus ofclaim 30 further including drive circuitry having an input coupled tothe sensor and an output coupled to the correction scanner, the drivecircuitry being responsive to the sense signal to produce an drivesignal.
 40. The scanning apparatus of claim 39 wherein the drivecircuitry includes a frequency doubling circuit.
 41. An imager foracquiring data corresponding to a target object, comprising:a firstscanning assembly having a first optical input and a first scan signalinput, the first scanning assembly being configured to scansubstantially at a first frequency about a first axis and to scan abouta second axis different from the first axis; imaging optics aligned tothe first scanning assembly and configured to collect light from thetarget object direct the gathered light along an optical path includingthe first scanning assembly; and a correction scanner positioned alongthe optical path and configured to redirect the gathered light along thesecond axis at a frequency and amplitude corresponding to an expectedamount of scan of the first scanning assembly about the second axisduring a half period of the first frequency.
 42. The imager of claim 41wherein the first scanning assembly includes a biaxial scanner.
 43. Theimager of claim 42 wherein the correction scanner scanner is a MEMsscanner.
 44. The imager of claim 42 wherein the biaxial scanner is aMEMs scanner.
 45. The imager of claim 41 wherein the first scanningassembly includes a pair of uniaxial scanners.
 46. The imager of claim45 wherein the correction scanner scanner is a MEMs scanner.
 47. Theimager of claim 41 for use in reading symbols, further comprising:aphotodetector oriented to detect the light redirected by the correctionscanner, the photodetetor being of a type that produces an electricalsignal indicative of the intensity of detected light; controlelectronics coupled to the photodetector and responsive to theelectrical signal to identify information represented by the symbol.